1. Field of the Invention
This invention generally relates to systems configured to inspect a wafer.
2. Description of the Related Art
The following description and examples are not admitted to be prior art by virtue of their inclusion in this section.
Inspection processes are used at various steps during a semiconductor manufacturing process to detect defects on wafers to promote higher yield in the manufacturing process and thus higher profits. Inspection has always been an important part of fabricating semiconductor devices. However, as the dimensions of semiconductor devices decrease, inspection becomes even more important to the successful manufacture of acceptable semiconductor devices because smaller defects can cause the devices to fail.
Improved sensitivity to particles, anomalies, and other defect types, while maintaining overall inspection speed (in wafers per hour), is desired in wafer inspection systems. Dark field optical inspection systems typically use laser light to illuminate the wafer in a specific pattern—individual spots, lines, or areas—and collection optics to direct the scattered light on a corresponding set of sensors.
One advantage of an inspection system where a large area (on the order of 1 mm by 1 mm) of the wafer is illuminated at once, as opposed to a spot (on the order of microns) or a line (on the order of microns wide by mm long), is that there exist many varieties of two-dimensional sensors that can capture information on thousands to millions of individual detectors in parallel. Furthermore, spot-illuminated inspection systems are practically limited to dozens of spots due to the complexities of illumination optics and of integrating individual sensors thereby limiting achievable throughput. One further disadvantage of spot and line scanning systems is the illumination energy is concentrated in relatively small areas, increasing the power density on the inspected surface, which can undesirably alter the sample properties.
It is well known that an XY (or serpentine) inspection sequence offers lower inspection throughput than a spiral sequence; therefore, a spiral trajectory (commonly known as R-Theta) is desirable under some circumstances. Examples of spiral inspection systems include the SP1 and SP2 instruments, commercially available from KLA-Tencor Corporation, Milpitas, Calif.
Despite the advantage of area inspection systems, as described above and in the art (e.g., U.S. Pat. No. 7,286,697 to Guetta), implementation of this configuration on an R-Theta platform has proven challenging, as there is an inherent mismatch of the spiral sequence of generated images with the rectilinear nature of most two-dimensional array sensors. Detection of defects by the aligning and registration of polar images in real time is a computationally intensive activity. Furthermore, the additional noise added to the measurement by most two-dimensional silicon-based sensors, as compared to discrete detectors such as photomultiplier tubes (PMTs), has in practice reduced the sensitivity performance of such systems. On XY-based area inspection systems, the coordinate mismatch problem does not exist, but previous embodiments of such systems have not been able detect all defects of interest at high speeds due to lack of flexibility of the illumination and collection subsystems.
Accordingly, it would be advantageous to develop inspection systems and/or methods that do not have one or more of the disadvantages described above.